A number of materials are particularly susceptible to accumulating a static charge. In the food packaging field where paper and plastic sheet or film are processed for use as packaging materials, sometimes as a laminate of paper and plastic are formed into a container for food or electronic chips and boards, the moving web can accumulate a large static charge. The sheet or film and the food contents being packaged can exhibit mutual repulsion or attraction thereby causing the food to bend away from the container. Further, this build-up of static electricity can interfere with the processing steps by clinging to the equipment, can attract dirt and contaminants from the air, and can cause electrical shock and fire. As a general rule the difficulties with static charge are greater as the relative humidity decreases. These materials vary in their susceptibility to accumulate a static charge, and to their rate of decay or dissipation of the charge.
Paper and plastic have a relatively high surface resistivity, and the charge decay for these materials occur at a relatively low rate. It is common practice in the art to reduce the accumulation of the static charge by increasing the rate of dissipation of the static charge such as by utilizing an antistatic agent. An effective agent must act at a relative humidity of below 40% and preferably below 25%. In accordance with the Static Decay Test, MIL-81 705 C, materials are tested at 15% relative humidity, and some industries use or require a test at 10% relative humidity. For these materials, the object is to achieve static decay similar to that of aluminum film or foil. Thus, according to this test, the substrate should exhibit a static decay of at least 90% in less than 0.2 second when measured at +5,000 volts and -5,000 volts, at 15% relative humidity, and 24 hours conditioning.
Synthetic hectorite clay, which is commercially available as "Laponite.RTM." from Southern Clay Products, Inc., a subsidiary of Laporte Industries Ltd., is a natural swelling clay useful as a flow control agent, and more recently has been known to be used as an antistatic agent. Naturally occurring hectorite contains contaminants which are difficult to remove, and therefore synthetic hectorite was developed in the early 1960's as a pure form. It is shown in the art that the synthetic hectorite clay useful as an antistatic agent has a portion of the hydroxyl ions replaced by fluoride ions. Laponite.RTM. B and Laponite.RTM. S are fluoride containing hectorites marketed commercially by Southern Clay Products, Inc. We have found that hectorite clays not containing fluoride ions are also useful antistatic agents.
Two general procedures have been employed in the art to Impart antistatic properties to various plastics. The first procedure involves the inclusion of various inorganic minerals, including synthetic hectorite, into the plastic prior to forming It into a film. The second approach has been to coat plastic films with a coating composition containing various inorganic materials, including synthetic hectorite.
There are many problems associated with coating films in order to provide a protective layer which has desirable properties, and certain of these disadvantages are set forth in U.S. Pat. No. 4,786,588. As is disclosed in this patent, protective layers containing inorganic substances can have insufficient covering properties and the surface of the layer can become rough and/or the coating can flake off during use. This prior art patent discloses one approach toward solving these problems in that the protective layer comprises a swellable silicate, a silanol compound, and an interlayer ion.
There is disclosed in the prior art in Japanese publication H3-169540 that the plastic film can be coated with a mixture of synthetic hectorite clay and kaolin, and an emulsion based adhesive having anti-blocking potential and Imparting adhesivity to the film. The resulting coated film exhibits improved properties, in particular antistatic and anti-blocking properties. According to the Japanese publication, these properties are not achieved without the mixture of hectorite and kaolin, together with the binder, as the coating composition.
The use of synthetic hectorite as a coating is disclosed in U.S. Pat. No. 4,173,480 for photographic sheet having a layered structure of paper, plastic, and a light-sensitive emulsion. According to this patent, photographic sheet material is prepared by sizing paper with gelatin, starch or carboxymethyl cellulose, then coating with a synthetic hectorite clay, specifically Laponite.RTM.S, and then applying a polymeric film which is corona treated to enhance the surface adhesion for the outer layer of a light-sensitive emulsion.
U.S. Pat. No. 5,429,867 and U.S. patent application Ser. No. 08/154,322 both assigned to the assignee of this invention disclose synthetic hectorite coated film and a two-component air-curable composition comprising hectorite clay, a peptizer and an epoxy ester.